The use of hydrostatic drive systems for agricultural and other work machines has been long established. The hydrostatic drive utilizes the substantially incompressible pressure of hydraulic fluid to variably drive a hydraulic motor with a variable volume hydrostatic pump. The application of this drive to agricultural vehicles is particularly useful in windrowers. By having a dual path, hydrostatic drive operating wheels at outboard portions of the windrower, a maximum of maneuverability is achieved at the end of a field harvesting to achieve minimum turning radiuses. While such a feature adds to the maneuverability of a hydrostatically driven windrower, the variations in pump output can have an impact on the ability of the windrower to track in a straight line and to accelerate in a uniform fashion. This is caused by manufacturing variations in the output of the individual pumps so that one may be more or less the output of the other at given field conditions or forward speed.
It has been a customary practice in the past to adjust the input for hydrostatic pumps by adjusting the overall linkage of a control rod connected between an operator steering and forward motion mechanism and radial arms used to vary the output of the hydrostatic pumps. While this may match the output of the pumps at a given pump output r.p.m., it does not necessarily do so over the entire operating range of the hydrostatic pumps.
What is needed, therefore, is a hydrostatic drive system providing uniform tracking, steering, and maximum speed.